I state reasons from 7 points.
NO.1 server location
As know as we can,Distance from your target country to your server location influence the speed of your speed.if your website face the global,you can chose server close to your target country.You can take care of USA or HongKong.
NO.2 the speed of customer visiting
Generally speaking,There are two factors affecting the speed.One thing,the website itself.The other thing configuration of server.If you have rich fund,you can buy speeder.
No.3 stability of server
Although some website have good speed upon visiting website,sometimes your clients can get access to your website.Even though you make a best optimization,maybe your website will result at the end of search engine even disappearing.That is why we should pay more attention to table server.
NO.4 network line and bandwidth for server
Network line get relation with stability of server.Good network is able to change smartly line,which ensure network smooth.You should figure out bandwidth is shared or private in where affect the performance of website running.
No.5 the size of IIS connection
Firstly,I explain the size of IIS connection to you.The size of ISS connection is the amount of visitors viewing your website at the same time.I imagine now you take it into your consideration when you chose server hosting.
NO.6 the parameters of servers
I suppose on this point i neednt to indicate it.
NO.7 scalability of server.
You website grow up in the long term.you need big server until your website large enough.so you ask hosting company that they support scalability or not.It is crucial in the future.