Your Org has tens of thousands of users, tens of millions of records, or hundreds of gigabytes of total record storage, have to follow best practices. Few Considerations:

Skinny Tables
Recommended to make your query fast. Important for batches taking long time to execute.

Indexes
Use index fields in where clause of Query. Multiple indexed field in query make it more faster. Even use Two-Column Custom Indexes. Two-column index is more efficient than single indexes on same fields.

Divisions
Partitioning the data of large deployments to reduce the number of records returned by queries and reports. For example, a deployment with many customer records might create divisions called US, EMEA, and APAC to separate the customers into smaller groups that are likely to have few interrelationships.

Mashups
One approach to reducing the amount of data in Salesforce is to maintain large data sets in a different application, and then make that application available to Salesforce as needed.

Deleting Data
Delete unused or unnecessary data. Use Bulk API's hard delete option, which allows records to bypass the Recycle Bin and become immediately available for deletion.

Search
When large volumes of data are added or changed, the search system must index that information before it becomes available for search by all users, and this process might take a long time.

Indexing with Nulls
Idea to reduce Null values, replace with NA or blank. Either use formula field to set value and index formula field.

API Performance
Add addition filter in rest API's query, so they dont scan all salesforce data.

Sort Optimization on a Query
Add a limit or sort order in query to make it fast and scan a limit of data only.

1. Checkbox
2. Date
3. Date and time
4. Email
5. Number
6. Percent
7. Phone picklist
8. Picklist (multi-select)
9. Text
10. Text area
11. Text area (long)
12. URL

The Concurrent Apex Limit exceeded issue is big issue in large data Orgs or implementations without following best practices. Exception generally received by end user or Admin:

"Unable to Process Request. Concurrent requests limit exceeded.
To protect all customers from excessive usage and Denial of Service attacks, we limit the number of long-running requests that are processed at the same time by an organization. Your request has been denied because this limit has been exceeded by your organization. Please try your request again later."

Salesforce Developer Blog Says:

The multitenant Force.com platform uses governor limits to ensure that system resources are available to all customers and to prevent any one customer from monopolizing them. If a governor limit is exceeded, the associated execution governor limit issues a runtime exception that cannot be handled. When you design your applications, you can help plan for their growth by keeping these limits in mind.

One of the limits customers frequently reach is the concurrent request limit. Once a synchronous Apex request runs longer than 5 seconds, it begins counting against this limit. Each organization is allowed 10 concurrent long-running requests. If the limit is reached, any new synchronous Apex request results in a runtime exception. This behavior occurs until the organization’s requests are below the limit.

Ultimately, this limit is in place to protect the user experience. Would you want your users waiting 5 seconds or more for a page to load?

So how would you know which operation is taking time? Even you can track it through Debug logs using many filter Extensions like 'Salesforce Debug Logs Analyzer'. One more level is Salesforce Event Log, obviously you should have access of it.

Go to https://salesforce-elf.herokuapp.com/

Login with your Salesforce Sandbox or Production login. Select dates range and apply, now you can download Salesforce Event Log File by Event Type.

Open in excel and Sort by Execution Time and see where its taking time, optimize that using best practice or adjusting business rules.

If the cause was a high callout time, you can filter your ApexExecution file based on QUIDDITY (E, H, M, R, V, W, X, L, K, or I) and CALLOUT_TIME (>5000), both of which were added in the Spring '16 release.

The values for quiddity in the logs include:

A - Legacy Batch Apex
C - Schedule Apex
E - Inbound Email Service
F - Future
H - Apex REST
I - Invocable Action
K - Quick Action
L - Aura
M - Remote Action
Q - Queuable
R - Synchronous
S - Serial Batch Apex
T - Apex Tests
TS - Synchronous Apex Tests
TA - Asynchronous Apex Tests
V - Visualforce
W - SOAP Webservices
X - Execute Anonymous

The DISTANCE function is now supported with location-based queries. You can calculate the distance between two location values, such as between a warehouse and a store.

DISTANCE() now works with location and address fields. You can retrieve a specific distance (say, of a store from a contact’s location) and display it to the user.

For example: With custom development, geocodes can unlock a whole new way to understand your customers and run your business. Here are some cool use cases that could be solved using geocodes: 

Now you can edit your existing chatter posts containing files or links. Before Summer 16 you have to comment if you missed mentions in posts.

Follow steps below to enable this feature:

• Setup | 
• Build | 
• Customize | 
• Chatter | 
• Chatter Settings |
• Select Allow users to edit posts and comments check-box.

In Summer 16, now you can see record details on hover in List view.

To enable Responsive List Hovers, follow below steps:

• In Setup, select Create | Apps, and find the name of the console you want to modify.
• Click Edit.
• Select Enable Responsive Lists.
• Click Save.

See preview without opening record!

In Summer 16 Global Picklist and Strictly enforce picklist values functionality is added!

Global Picklist can added by following steps:

• From Setup, enter Picklists in the Quick Find box, then select Picklists.
• Next to Global Picklists, click New.
• Enter a label for the global picklist. This name appears in Setup, and when users create a picklist based on this global picklist.
• To tell admins what this set of values is for, enter a specific description of the global picklist. This text appears on the Picklists list page in Setup.
• Click Save.
• In the Picklist Value Set section, click New.
• Add picklist values by clicking New, then entering the values one per line.
• Click Save.
• To reorder the values, click Reorder. Optionally, choose to sort values alphabetically or to use the first value as the default value.

Use to comparing the field on related object.

Syntax:
VLOOKUP(field_to_return, field_on_lookup_object, lookup_value)

Create a validation rule and pass parameters, field_to_return, field_on_lookup_object & lookup_value.

Field_to_return : The field that contains the value you want returned
Field_on_lookup_object : The field on the related object that contains the value want to match
Lookup_value : The value want to match.

For example:

Country(1st object):

Fields:

Name(Text)
Capital(Text)

Customer(2nd object):

Fields:

Queueable Apex is essentially a superset of future methods with some extra features, released in Winter 15. We can say its a mixer of future methods and the power of Batch Apex! It is called by a simpleSystem.enqueueJob() method, which returns a job ID like Apex batch that you can monitor.
Queueable Apex allows following additional benefits:

TEMPLATE NOT FOUND issue due to non access of field in email Templates. Salesforce is saying that Email Template is using standard controller even for Visualforce templates using without sharing classes. So you can't send email through template if user don't have access of fields used in template!

Send email from Apex is counting against limit of email send so its necessary to send email through email template by workflow. We tried a work around and its working fine. Follow following steps:

1. Make a field on object from where email need to send.
2. Make a workflow that send email on change of above field you made.
3. Use email template that contains above made field in template.
4. Populate field when condition matched or need to send email.