Drupal 7 performance related patches & replacements for core functionally

Core Patches

Big Performance Gains - Low Risk

Second loop in module_implements() being repeated for no reason. ~ 400ms - Included in 7.40
Speed up drupal_parse_info_format() 3x and reduce memory 95% ~ 600ms - Included in 7.40
Add static cache to module_load_include() ~ 45ms - 1.1s - included in 7.42
improve theme registry rebuild performance by 85% ~ 4 seconds Included in 7.51 October 2016

Small Performance Gains - Low Risk

inline file_uri_scheme() in file_stream_wrapper_uri_normalize() ~0-150ms Included in 7.x-dev September 2016
Avoid re-scanning module directory when multiple modules are missing ~0-300ms included in 7.50
Add static cache to DatabaseConnection ::escapeTable(), ::escapeField() and ::escapeAlias() ~20-100ms with large number of similar queries (commerce line items). Included in 7.x-dev September 2016

Small Performance Gains - High Risk

If item is hidden in _menu_tree_check_access() skip it right away ~5-50ms

Race Conditions

Fixes for MySQL Deadlocks in D7
Notice: Trying to get property of non-object in image_style_deliver()

Fix Bugs

Fix PHP Notices

Control Memory Usage

Contrib Patches

Link Module - Only re-load the entity in _link_sanitize() if tokens need to be replaced IN DEV branch v1.3
Context Module - Cache query in context_reaction_block::get_blocks() & remove unused blocks before the alters get called
Panels Mini Module - panels_mini_load & friends need caching
Workbench Moderation Module - Node revision history optimization on large websites
Menu Item Visibility Module - menu_item_visibility_load time

Performance

Replace Core's Functionally

Cache Backends

Memcache API and Integration (cache, locks, sessions)
Redis (cache, locks, sessions)
Entity cache (load entities from cache rather than DB)
FileCache No special server requirements so suitable for shared hosting.
MongoDB (cache, locks, sessions, field storage, watchdog, block, queue)
Book Cache Memory and query optimizer for book module in rendering capability.

Database

AutoSlave (automatic use of slave db, scalability)
APDQC (tuning guide / optimizations beyond default mysql controller), also provides a lot of optimizations previously described above.

Page

Cache Expiration - configurable actions upon events that will expire URLs from caches
Boost - serve static pages to anonymous users as single files / db hits.
AuthCache - serves authenticated requests faster than stock drupal serves anonymous requests.

Statistics

jStats
Google Analytics Statistics

Block/Pane Alt Rendering

Ajax Blocks
Edge Side Includes integration

HTTP Requests

chr
HTTP Parallel Request Library - Replacement for drupal_http_request()

CSS/JS file aggregation/optimization

Advanced CSS/JS Aggregation
Speedy
Magic
Aggregate cache
Core Library
UglifyJS
Simple aggregation

Others

Tracelytics - tracelytics web application performance analysis Drupal integration
Menu Performance - Benchmarks: ~200ms improvement with 1000 menu items.
Big Autocomplete TAXonomy - Use autocomplete form field if taxonomy is bigger than a predefined number of terms.
Imageinfo Cache - Image style pre-generation.
Render Cache - Backport of D8 render caching engine, requires a lot of tuning / experimental.
Display Cache - Weaker version of render_cache, has potential side effects.

Drupal 6 wiki

https://groups.drupal.org/node/187209