I just check Skeeter site the ZX,FX and 20I are all 20'2 long

They are different weights tho

Don't different air temps and altitudes make outboads run different speeds? I've seen somewhere that the difference between hot summer air and cold winter air is a couple miles per hour. (I guess it would actually be running quicker in colder air versus warm air)

I run a 2010 Fx20 with a 250 Sho. The fastest it has ever ran with me by myself was 76 full fuel and tourney load. With two people I get around 72. I think it all has to do with the temp, humidity and wind.( I could be wrong, I am by no means an expert ) If your rpms have changed there is definitely something wrong.

The 19.6 ft boat is the ZX 200 series that they rate for a 200hp motor. Zx225 was 20'2". And your zx20 should be 20'2". My boat is a 2000 and was the last year of the zx202 which is the same platform as your zx20 at 20'2" BUT my boat is only rated for a 200. I have a turtle compared to yours. About 64-66 is max for me

I have ZX 200 with a 200 series 2 summer time I run around 65 cooler months about 67-68.

My '05 zx225 runs 72 top end

Mine has. My ZX 21 in the first hour after break in did 77 mph. a few times. I have noticed since that time, it has leveled off to about 74 mph. Motor now has 9 hours on it, and it averages about what Mark said, 74 on top speed. But I normally dont take it above 70.

My ZX 20 first ran about 75 mph, now it runs about 70 mph. Now, this could be due to fact that I didn't have all of my tournament [censored] in it the first time. I hadn't had time to load it all up. Way too much stuff. Clean it out Steve.

You are talking about a 10MPH difference. Definitly a problem somewhere. 1st: you are talking GPS speed right? 2nd: are you still reaching the same RPM's as before? And 3rd: are you getting it trimmed up properly? This actually sounds like your problem because you are saying it "stops trimming up at a certain point", right? No reason for your trim to not keep raising until you actually have have it "over trimed", cavitating, overheating and coming completely out the water.

I have a 2016 zx250 the highest speed I have achieved is 74.2, but on average I am getting about 72.5

My FX21 with 250 SHO has never run higher than 67 at 5,600 RPM
Jack plate at 4" and trimmed 78%

Sure you just haven't added more weight in boat since you got it? I can run 75 but depends on how much gas I got too. Plus depends on the water if choppy or smooth.

You drive like a grandpa!

from your analysis seems there has been some weight added--either you have gainded a lot of weight or added more fishing stuff...