Unless you have a chrony it's pretty hard to tell. Because of the difference in the feel of the gun, it quite often seems to be less powerful but in reality, it's not.
The heavy tar if used in excess can reduce the velocity, especially in cooler to cold weather. Also, a lot depends on what spring you used and what seal. Another factor could be the seal sizing if needed. If it is pretty tight, it can cause some excessive drag. Also, if you were not careful in the deburring process, you may have nicked the seal. Normally the .177 should be in the 850-860's to the low 900's somewhere depending on the pellet and how it is set up. I don't really like to see them upwards of 920 or so and sometimes detune them to drop them down a little.