I'm working on a BB game and I have a question about its performance. Let's say I have 1000 users and using cronjobs every 15mins I update user set ore=ore+'20'; and every hour I update user set turns='20';

Will this have any significant effect on the performance of my game?

I don't think anyone could provide anything other than speculation based on the limited information that you have provided. Whether additional workload such as a Cron job will affect performance will depend on how close to the limit of the server you are and how efficient the Cron jobs are. The first part, the server limit will depend on a lot of factors. That includes the type of hardware you are using, if it is dedicated to this application, the efficiency of the application, the number of transactions being processed at one time and so forth. If you have 1000 users, some % of those will be active at one time. In any given second, you will only receive transactions from a subset of those users so knowing the average and peak arrival rate is important information.

To do this properly requires some modelling and benchmarking. If you are far from having a problem, some quick analysis might demonstrate that you are ok. If you are close to the line, then more serious analysis will be needed.