iPhone Simulator - Simulate a slow connection?

It also worth mentioning that Xcode also has a built in way for devices, not simulator.

• Just go 'Devices and Simulator' (cmmd+shift+2)
• Select your device
• Scroll down til you find 'Device Conditions'
• Set your desired profile
• Hit Start

To have this working you need to install 'Network Link Conditioner' on your Mac. See steps mention in Alan's answer

How to install Apple’s Network Link Conditioner

These instructions current as of October 2019.

Warning: If you just upgraded to new version of macOS, make sure you install the very latest Network Conditioner (in Additional Tools for Xcode) or it may silently fail; that is, you will turn it on but it won’t throttle anything or drop any packets.

Update: As of Xcode 11, there may be an even simpler way to simulate network conditions on tethered devices; see this blog post. For how to affect simulated devices, continue below, as before.

1. Install Xcode if you don’t have it.

2. Open Xcode and go to Xcode › Open Developer Tool › More Developer Tools…

3. Download Additional Tools for Xcode (matching your current Xcode version)

4. Open the downloaded disk image and double-click the Network Link Conditioner .prefpane under “Hardware” to install it.

5. There we go!

6. Be sure to turn it on. You need to select a profile and enable the network conditioner.

Caveat

This won't affect localhost, so be sure to use a staging server or co-worker's computer to simulate slow network connections to an API you’re running yourself. You may find https://ngrok.com/ helpful in this regard.

An app called SpeedLimit

https://github.com/mschrag/speedlimit

Works great.

chris.

"There's an app for that!" ;) Apple provides "Network Link Conditioner" preference pane that does the job quite well.

• for Xcode versions prior to 4.3, the pane installer can be found in your Developer folder, e.g. "/Developer/Applications/Utilities/Network Link Conditioner", after installation, if daemon fails to start and you don't want to reboot your machine, just use sudo launchctl load /system/library/launchdaemons/com.apple.networklinkconditioner.plist
• if you are already done with Developer folder, you can install the pane as a part of "Hardware IO Tools for Xcode" package available via Mac Dev Center additional downloads section.

Link to download page (you must log in with your Apple ID): https://developer.apple.com/downloads/index.action
(credits to @nverinaud)