Well, the web has been around forever - actually in Xe there were web services - but it was horribly unreliable until around SP21 or SP22 or so. To the degree that most customers didn't even touch web until they were forced to.
In 8.10, users were able to use Fat Clients (citrix) still - but it was strongly recommended that they started using web. Powerforms were introduced - specifically P42101 was introduced with 8.10, which required web (ie, does not run on a fat client).
By 8.11, customers were forced to run web for the apps, as JDE disabled access to the majority of the Win32 "fat" client applications - and forced users to adopt to the web.
Of course, the web has changed so significantly over the past few years as well. Early (Xe/8.0) service packs used "jrockit" to power the J2EE engine. Then Websphere was introduced as the "web server of choice" - and weblogic was available waaaay back when. When Peoplesoft bought JDE, they disabled the weblogic, and concentrated on websphere. But when Oracle bought Peoplesoft (and JDE) - they introduced OC4J (Oracle Application Server) as an engine. Only by 8.98 or so did Oracle finally subscribe to Weblogic for JDE as the main J2EE server type - and only with 9.2 (the latest) - did they finally get rid of any trace of OC4J !
Websphere is still an option to use, but the majority of customers (including many iSeries customers I work with) use Weblogic as they purchase the JDE Technology Foundation license.
Now - as far as performance of JDE compared to the web ? I'd say the Web "matched" the fat client around 8.12. Certainly with reliability. Up until then, the fat client was not the better performing version. Lastly. Citrix still is the lowest bandwidth/better latency/easier to manage option - so there are customers that prefer to deploy Internet explorer through citrix - but this is moving away these days as an option, especially as other browsers (chrome/safari) are a LOT easier to lock down. We all hate ActiveX controls, and luckily you can disable ActiveX easily these days....