MySQL has no boolean field type at all even there is one. This …
… because the existing type boolean is a tinyint behind the hood (at least in MySQL 5.anything).

According to my linked observations the index in mlf2_b8_rating seems to work while the index in the similar structured table mlf2_akismet_rating does not work.

Yes to all this, and very interesting on your observation between the 2 tables (b8_rating vs akismet_rating). Let me do a little more digging here.

FYI, Here's some performance comparison between the versions I'm testing. All on the same server, but each has an identical copy of the same DB (with upgrades). Many runs of each to mitigate shared server performance issues.

1. Performance of forum running MLF 2.0.2

• 1. Home Page (20, 15), not logged in: 2 - 2.25 seconds
  
• 2. Home Page (20, 15), logged in: 2.25 - 3 seconds
  
• 3. Admin Page, logged in: 1 second
  
• 4. Contact Page, not logged in: 1 second

2. Performance of forum running MLF 2.3.5

• 1. Home Page (20, 15), not logged in: 1.75 - 2.5 seconds
  
• 2. Home Page (20, 15), logged in: 2.25 - 3.25 seconds
  
• 3. Admin Page, logged in: 1 second
  
• 4. Contact Page, not logged in: 1 second

3. Performance of forum running MLF 2.4.24

• 1. Home Page (20, 15), not logged in: 4 - 5.5 seconds
  
• 2. Home Page (20, 15), logged in: 0.75 - 1.5 seconds
  
• 3. Admin Page, logged in: 0.5 seconds
  
• 4. Contact Page, not logged in: 0.5 second

4. Performance of forum running MLF 20220803.1

• 1. Home Page (20, 15), not logged in: 5.5 - 6.75 seconds
  
• 2. Home Page (20, 15), logged in: 3 - 4 seconds
  
• 3. Admin Page, logged in: 3 seconds
  
• 4. Contact Page, not logged in: 3 seconds

2.4.24 is a combination of the best and almost worst for performance. Main page is fantastic when logged in, but when not logged in, performance suffers dramatically. This is also impacted in 20220803.1, but not earlier versions. I'm going to take a deeper look at the differences in queries when logged in vs not.