Zooming and panning slows down on at lower zoom levels with if a lot of data is loaded

[contrapunctus]

What steps will reproduce the problem?

1. Download using query (highway=bus_stop or (public_transport=platform and bus=yes)) or (type:relation and type=route and route=bus) in Delhi
2. Try to zoom or pan at a low zoom level

When working with PT data, this is my preferred workflow, because it

• gives me access to all route relations at one go without further interruption in work,
• avoids duplication of route relations,
• lets me see bus stops which are not part of relations.

URL:https://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2018-03-26 22:16:39 +0200 (Mon, 26 Mar 2018)
Build-Date:2018-03-26 20:45:23
Revision:13576
Redirecting:to URL 'https://josm.openstreetmap.de/svn/trunk':
Relative:URL: ^/trunk

Identification: JOSM/1.5 (13576 en) Linux Debian GNU/Linux 9.4 (stretch)
Memory Usage: 374 MB / 866 MB (168 MB allocated, but free)
Java version: 1.8.0_171-8u171-b11-1~deb9u1-b11, Oracle Corporation, OpenJDK Server VM
Screen: :0.0 1366x768
Maximum Screen Size: 1366x768
Java package: openjdk-8-jre:i386-8u171-b11-1~deb9u1
Java ATK Wrapper package: libatk-wrapper-java:all-0.33.3-13+deb9u1
Dataset consistency test: No problems found

Plugins:
+ alignways (34109)
+ continuosDownload (68)
+ download_along (34081)
+ pt_assistant (34117)
+ public_transport (34109)
+ reverter (34109)

Map paint styles:
- https://github.com/bastik/mapcss-tools/raw/osm/mapnik2mapcss/osm-results/mapnik.zip
- https://josm.openstreetmap.de/josmfile?page=Styles/PublicTransport&zip=1

Last errors/warnings:
- W: No configuration settings found.  Using hardcoded default values for all pools.

[Don-vip]

In 13875/josm:

see #16128, see #16251 - display street labels on z18+, operation very costly on urban areas

[taylor.smock]

Closed as duplicate of #10101.
See also #11487 (render data to tiles) which will improve the UI responsiveness drastically, at the cost of slower (initial) painting.