I have the same problem.
I develop a CI-based CMS. During my testing with 3.0.0, page loads have fallen down from 300ms to 1200ms.
It does not seem to be related to the database class. With full caching activated, I go down to 0 or 1 request per load, and it is still a lot slower.
I installed CI2 and CI3 on a local xampp server. No modifications at all, just the default welcome page.
I get an 18ms average with CI2 for page load, but a 32ms average for CI3.
I had been using the 3.0DEV branch for the past year without any problems. It seems that it is only in the last months that things got slower. I'll keep on troubleshooting and keep you posted if I find a cause.
Should we open a GitHub issue or is this too vague yet?
