When using a session, requests are slower than in previous versions.
My metrics indicate a performance degradation, and after some testing I suspect this is the root cause.

Expected Result

Same performance as in version 2.25

Actual Result

When I use a session, requests are about 70% slower than in previous versions.
This seems to be the PR that caused the regression: #5681
I removed the rebuild_proxies call locally and performance shot back up.
I ran a benchmark on 1000 requests making a real-world request with the same session.
In version 2.25.1 it took ~13s, in version 2.26.0 it takes ~23s.

System Information

$ python -m requests.help

{
  "chardet": {
    "version": "3.0.4"
  },
  "charset_normalizer": {
    "version": "2.0.3"
  },
  "cryptography": {
    "version": "3.4.7"
  },
  "idna": {
    "version": "2.10"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.7.10"
  },
  "platform": {
    "release": "5.4.0-1055-azure",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "101010bf",
    "version": "19.1.0"
  },
  "requests": {
    "version": "2.26.0"
  },
  "system_ssl": {
    "version": "101000cf"
  },
  "urllib3": {
    "version": "1.26.6"
  },
  "using_charset_normalizer": false,
  "using_pyopenssl": true
}

This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c).