I have very slowly site, but it is T2large AWS EC2,(about CPU only 30% used on average) I have 5,9 sec TTFB (I try cloudflare, but TTFB stil about 6 sec)
You can check TTFB test or google test speed.
Goal is TTFB have under 300ms - (now 5-6 sec) I use Yii2 framework PHP, on AWS Ec2 server. EBS IOPS have now increased to 1500, maybe do it EBS (low IOPS) am not sure. - because I see on cloudwatch (Burst Balance EBS is average 100%) I think it is on side server (TTFB) but I am not sure.