Making website lagg

I bought the full version on the plugin on 5 March 2015 after using the free version since Nov 2013.
Since using this paid plugin my website has been lagging and even bring my whole server down that is hosting a few other websites as well. This is happening more and more often. It started happening once a week or so and now it is daily.
I did notice that since I created a new booking form that I use for some of my events this also increased the occurrence of the the lag/downtime.

Please look into this and see if there is any caching that can be causing this. We have about 150 000 page view a month and this is becoming a real problem for our company to have our sites down everyday.

https://www.ads-software.com/plugins/events-manager/