OnlineClient is the name of a Mac app that you can discover installed on your Mac, usually without any recollection of installing it. OnlineClient exhibits traits that are commonly found in adware.
OnlineClient is an application with a generic-looking icon - a dark gray circle with a white looking glass icon inside it. Several other similar apps displaying very comparable adware behavior share the same icon.
OnlineClient will display unsolicited advertising inside your browser and can inject ads into the web pages that you browse. This behavior is typical for adware or ad-supported software. The operators of the software make deals with ad networks to deliver what is essentially unsolicited ads inside the user's browser.
The issue here is not just that the ads are slowing down your browser. Some of them can also be sourced from rogue advertising networks and link to unsafe pages and questionable content. All of those features make apps similar to OnlineClient an undesirable presence on your Mac computer.
In most cases, this kind of app can simply be uninstalled. In cases where the undesirable app has some sort of persistence mechanism, you may need to use an anti-malware suite to completely get rid of it.