How to choose which JUnit5 Tags to execute with Maven

You can use this way:

<properties>
    <tests>fast</tests>
</properties>

<profiles>
    <profile>
        <id>allTests</id>
        <properties>
            <tests>fast,slow</tests>
        </properties>
    </profile>
</profiles>

<build>
    <plugins>
        <plugin>
            <artifactId>maven-surefire-plugin</artifactId>
            <version>3.0.0-M5</version>
            <configuration>
                <groups>${tests}</groups>
            </configuration>
        </plugin>
    </plugins>
</build>

This way you can start with mvn -PallTests test all tests (or even with mvn -Dtests=fast,slow test).

Using a profile is a possibility but it is not mandatory as groups and excludedGroups are user properties defined in the maven surefire plugin to respectively include and exclude any JUnit 5 tags (and it also works with JUnit 4 and TestNG test filtering mechanism).
So to execute tests tagged with slow or fast you can run :

mvn test -Dgroups=fast,slow

If you want to define the excluded and/or included tags in a Maven profile you don't need to declare a new property to convey them and to make the association of them in the maven surefire plugin. Just use groups and or excludedGroups defined and expected by the maven surefire plugin :

<profiles>
    <profile>
        <id>allTests</id>
        <properties>
            <groups>fast,slow</groups>
        </properties>
    </profile>
</profiles>