Pretty much every manager who has managed a Prem team in Europe has said how much of an impact the extra games have had. If you think it is not a problem then West Ham have probably the best team that ever played in the Prem.
I am pretty sure we once played West Ham and lost on a Saturday lunchtime after playing Moscow away on a Wednesday and before that we played Chelsea away.
The impact of the extra games leads to more injuries and players becoming physically drained. The players have to mentally raise themselves more often. You also have less fitness and training sessions and sometimes only have a day or two to prepare for a game, while the opposition have had a week. Football at the top level is about fine margins. As soon as the players feel less prepared - physically, mentally and tactically - the likelihood of losing increases substantially. You will soon see.